Contrasting Patterns of Labile and Semilabile Dissolved Organic Carbon From Continental Waters to the Open Ocean

Marine and freshwater prokaryotes feed primarily on bioavailable labile dissolved organic carbon (BDOCL), as well as the bioavailable fraction of the semilabile DOC (BDOCSL) pool. These fractions are operationally defined here as the DOC consumed within a month and greater than a month to a year and a half, respectively. Organic matter from these different pools comes from various autochthonous and allochthonous sources, but their relative bioavailability is unknown across aquatic ecosystems. To fill this gap, we compiled literature information that included 653 batch culture DOC biodegradation experiments across eight aquatic ecosystem types over the past 20 years. We show that the proportion of BDOCL across all aquatic ecosystems was surprisingly consistent (6.1%) despite a 2 order of magnitude variation in initial DOC concentrations, suggesting an overall tight balance between carbon supply and consumption. A higher proportion of BDOCL, 16.3% on average, was observed in high productivity ecosystems. BDOCSL, on the other hand, gradually decreased from 16.0% in lakes to 7.2% in estuaries to undetectable in the open ocean, suggesting that terrestrial connectivity regulates BDOCSL across the continuum. Our results support that recent primary production fuels short-term prokaryotic DOC needs with an increasing reliance on the abundant BDOCSL pool as ecosystems approach the land-water interface. Batch culture experiments show that BDOCSL is metabolizable in freshwater and coastal environments but not in the open ocean. We estimate that BDOCSL can sustain 62% of total prokaryotic biomass in inland waters and coasts and an estimated total of 16.7% across aquatic biomes. Plain Language Summary Bacteria are the most abundant organisms in lakes, coasts, and oceans outweighing all the whales living in the ocean! But what do they consume? Bacteria eat organic carbon dissolved in water, but not all carbon compounds are equally easy to eat. Our aim was to understand the amount of carbon bacteria eat and how well different organic carbon sources support bacteria across lakes, coasts, and oceans. We found that the relative amount of readily available carbon to support bacteria is the same no matter what aquatic ecosystems they live in. It is also less than what was previously thought. Second, the more an aquatic ecosystem is connected to land, the more carbon was available since some of it comes from soils, but it takes over a month for it to be consumed. Therefore, the more connected to land the aquatic ecosystem is, like a lake, the more the bacteria can rely on that additional terrestrial source of food. Surface bacteria of the open ocean appear to rely solely on readily available, locally produced organic matter (carbon), whereas bacteria from lakes and coasts have access to both readily available and partially decomposed organic matter from local production and terrestrial resources. Key Points The relative amount of dissolved organic carbon consumed within 31 days by prokaryotes is similar across all aquatic ecosystems The relative amount of organic carbon consumed beyond 31 days decreases with reduced terrestrial connectivity along the aquatic continuum The standing stock of semilabile dissolved organic carbon may sustain 62% of the prokaryotic biomass in lakes and coasts